Severe haze in Hong Kong over the past two days has prompted a government warning of serious health risks, with experts urging the elderly and vulnerable to stay indoors until sunset.

Air quality health risk levels at some of Hong Kong’s roadside monitoring stations rose on Wednesday and Thursday, with experts attributing the surge to elevated ozone levels caused by sunlight and high temperatures and warning that conditions were likely to persist over the next few days.

On Wednesday afternoon, the Air Quality Health Index issued by the Environmental Protection Department reached a “serious” health risk level at its Tuen Mun station, where it remained for two hourly readings.

The pollution persisted on Thursday, with Central/Western, Southern, Tsuen Wan, Tuen Mun and Sha Tin stations recording “high” health risk levels at 4pm. An hour later, the Southern station reached a “very high” level. The hourly readings eventually began to fall in the evening.

Ho Kin-fai, a professor at the Chinese University of Hong Kong’s Jockey Club School of Public Health and Primary Care, said light winds had brought pollutants to the city, which, combined with high temperatures and sunlight this week, intensified photochemical smog activity -- the major factor leading to the formation of ozone.

“Ozone remains the primary reason for the high health risk readings in the past two days, although it has occurred in different locations,” Ho said. “The situation may likely persist over the next few days.”He said elderly and vulnerable people should remain mindful of air quality when the index is at the “high” health risk level.

Launched in 2013, the index tells residents of the short-term health risks posed by air pollution. It analyses three-hour average concentrations of four major pollutants, namely ozone, nitrogen dioxide, sulphur dioxide, and fine particulate matter – PM2.5 and PM10 – on a scale from 1 to 10, as well as 10+.

When the health risk level is “high”, the department advises children, the elderly and people with existing heart or respiratory conditions to reduce outdoor physical activity, especially in areas with heavy traffic. They should stay indoors when the index reaches the “serious” level.

For healthy adults who want to exercise outdoors, Ho suggested shortening workout durations or running later at night, as ozone levels gradually decline after sunset.

Air quality is usually worse in winter compared with other seasons due to meteorological conditions, said Leung Wing-mo, a former assistant director at the Hong Kong Observatory.

During winter, the northeast monsoon may bring air pollutants from mainland China over to Hong Kong, Leung said, though he added that air quality in the Pearl River Delta region had improved over the past two decades.

Wind speed also played a role, he noted, as stronger winds create turbulence that lifts ground-level pollutants high into the atmosphere.

Cold spells influenced by the northeast monsoon also tend to last a few days at a time, meaning conditions may ease periodically, according to Leung.

Cooler temperatures were another factor, he said. As cool air near the surface settles in, the denser, cooler air beneath the warmer air leads to a higher probability of “temperature inversion”, slowing the dispersion of pollutants and causing them to accumulate near the ground.

Leung said it was challenging to determine whether Tropical Cyclone Champi had affected Hong Kong’s air quality, given the presence of other competing factors.

However, he noted that because Champi was a “big and intense typhoon”, its potential effect on Hong Kong through subsiding air could not be ruled out.

According to the city’s forecaster, Champi will move towards the seas east of Taiwan on Friday, before it is likely to steer northwards towards Japan and Korea.

Under the influence of its subsiding air, weather along the Guangdong coast will remain very hot through the latter half of the week, the forecaster added.